The use of the sterically demanding IPr* and related ligands in catalysis

This account highlights the synthesis and applications of one of the very bulky NHC ligands, IPr* (1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-ylidene). This ligand and some of its derivatives have been found very effective in several catalytic applications and have enabled the isolation of highly reactive organometallic complexes. More specifically, applications of this ligand in Pd and Ni chemistry have permitted challenging transformations under mild reaction conditions and low catalyst loadings. We report the successes as well as the limitations encountered using transition-metal systems bearing this ligand-type. This report will hopefully serve as a guide to synthetic chemists, providing insights as to when the very sterically demanding IPr* ligand (and its congeners) and in a broader context, very bulky NHC ligands, should be used.